At the beginning of the game, the phone has only one number- Toriel's home phone. Calling this number yields different dialogue from Toriel depending on Kris's location:
- If called from Toriel's home, Toriel picks up but becomes agitated with Kris.
- If called from the main room, the landline rings. However, narration jokingly states that Kris cannot pick up the phone because they are already on their cell phone.
- If called from elsewhere in town, Toriel encourages Kris to walk home and reminds them of where her house is located. If called again, she mentions she is busy grading.
- Toriel calls Kris (alternatively, Kris may call Toriel) on the way out of school following their return from the Dark World. Toriel expresses worry, but is happy that Kris was spending the time with a friend and permits Kris to stay out a little longer.
- If called after returning from the Library's Dark World in Chapter 2, Toriel will ask Kris how their school project went and tell them to be home by sunset.
- If used in the Dark World, the Cell Phone makes the same sound as W. D. Gaster's entry 17 from Undertale,
mus_smile.ogg, and does not function.
- Susie can hear this sound in Chapter 2 while she and Kris are imprisoned in Queen's dungeon. Susie will worriedly ask if Kris's phone usually yells.
- By talking to Sans, Kris can obtain a second phone number. However, when called, it directs Kris to a hotline for idiot babies. The option then changes to "Not Sans's Number" and rings indefinitely when called.